Conventionally, in performing a mass production of the same kind of product, it is general to perform a series of process on an assembly production line. This production line is called a “conveyor production line”, and is widely employed for products such as automobiles. In the conveyor line system, it is very important to keep constant a processing time of each process included in the production line. When a certain process requires a longer time than that of other processes, this process becomes a bottleneck, and decreases the operating rate of processing devices used in other processes.
On the other hand, in the production line of semiconductor chips and the like, the same processing device is often shared in plural processes, unlike the conveyor line system. This production line is called a “job-shop production line”. In the manufacturing of semiconductor chips, for example, cleaning of wafers, film formation of conductors and dielectric substances, photolithography, and etching are performed repeatedly at many times.
Among the above processes, the wafer cleaning process rarely becomes a bottleneck, when the same processing device (cleaning device) is shared in plural processes, because many wafers can be collectively processed. The film formation process and the etching process require different conditions of process gas depending on materials to be formed and materials to be etched. Therefore, it is difficult to share the same processing device (a film formation chamber, an etching chamber) in plural processes. Consequently, in many cases, an exclusive chamber is used for each process of the film formation process and the etching process, and these processes seldom become bottlenecks. However, the film formation process and the etching process may become bottleneck processes when troubles occur in the devices.
On the other hand, the photolithography process usually requires a sharing of a stepper in plural processes, because a processing device (the stepper) to be used in the photolithography process is considerably expensive. Further, unlike the cleaning process, the photolithography process does not allow a batch process of many wafers, and requires the wafers to be processed one by one. Therefore, in the production line of semiconductor chips, the photolithography process becomes a bottleneck in many cases.
The processing device (the stepper in the above example) which becomes a bottleneck is sometimes shared to produce plural kinds of products. In this case, it is very difficult to determine which one of products in progress is to be processed first with priority among the products in progress accumulated before the processing device.
FIGS. 22A to 22C are one example of a job-shop production line for producing two kinds of products α and β. FIG. 22A is a schematic diagram showing processing devices to be used, FIG. 22B is a table of the processing devices to be used to produce the product α, laid out in the order of the processes, and FIG. 22C is a table of the processing devices to be used to produce the product β, laid out in the order of the processes.
As shown in FIG. 22A, the production line includes six processing devices A, B, C, D, E, and P, and a process to be executed by the processing device P is a key process. In this production line, as shown in FIG. 22B, six processes including a process 11 to a process 16 are performed to produce the product α, and the processing device P is used in the process 12, the process 14, and the process 16. As shown in FIG. 22C, four processes including a process 21 to a process 24 are performed to produce the product β, and the processing device P is used in the process 22 and the process 24.
In the above production line, many products in progress tend to be accumulated immediately before the key process performed by the processing device P. Therefore, it becomes necessary to determine which one of the products in progress should be processed with priority by the processing device P. In this case, as a method of determining the priority order, a First In First Out (FIFO) method and a Last Buffer First Served (LBFS) method are known. The former method, called the FIFO method, is a method of processing the products in progress, starting from the oldest product reaching the processing device. The latter method, called the LBFS method, is a method of processing the products in progress, placing a highest priority to the downstream process.
FIG. 23 is a table showing a transition of the number of products in progress when the FIFO method is used to determine the priority order.
In the example shown in FIG. 23, it is assumed that three lots of the product α are present immediately before the process 12, and two lots of the product β are present immediately before the process 22. Three lots of the product α are older products than two lots of the product β. The operation time of the key process using the processing device P is one day, and the operation time of the process using other processing device is zero day. In this case, when the priority order is determined based on the FIFO method, the products in progress shift as shown in FIG. 23. After four days pass, three lots of the product α are present immediately before the process 14, and one lot of the product β is present immediately before the process 22 and immediately before the process 24, respectively. In FIG. 23, the lots encircled by a round mark are the lots in progress (the same also applies to the subsequent explanations).
In this case, a work-in-process variance (σ) from the initial state until a lapse of four days is 1.2, and the number of finished lots is zero. As explained above, the use of the FIFO method has an advantage in that old stocks are processed with priority. On the other hand, as shown in FIG. 23, the use of the FIFO method has a disadvantage in that the products in progress shift in this order, and the number of products in progress is not equalized.
FIG. 24 is a table showing a transition of the number of products in progress when the LBFS method is used to determine the priority order.
In the example shown in FIG. 24, it is also assumed that three lots of the product α are present immediately before the process 12, and two lots of the product β are present immediately before the process 22. The operation time of the key process using the processing device P is one day, and the operation time of the process using other processing device is zero day. A priority order of the key process is defined such that the process 16 has a highest priority, and the process 24, the process 14, the process 22, and the process 12 have priorities in this order. In this case, the products in pr ogress shift as shown in FIG. 24. After four days pass, three lots of the product α are present immediately before the process 12, and two new lots of the product β are present immediately before the process 22.
The addition of two new lots is because, in the production line of semiconductor chips, what is called CONWIP (CONstant Work-In-Process) is performed in many cases, to start new production of a product by the number of finished products.
In this case, a work-in-process variance (σ) from the initial state until a lapse of four days is 1.44, and the number of finished lots is two. As explained above, the use of the LBFS method has an advantage in that finished lots are obtained at an early stage. On the other hand, the use of the FIFO method has a disadvantage in that only one kind of product (the product β in this example) is processed, and production of other kind of product (the product α in this example) is not progressed.
As a method of determining a priority order in the job-shop production line, a method described in Japanese Patent Application Laid-open No. H11-145021 is known. However, in the technique proposed in the patent document, a priority order is determined based only on the processing time of the process immediately after the key process which becomes a bottleneck. Therefore, a proper priority order cannot be determined when the processing device P to be used in the key process is used for plural kinds of products.